This invention concerns improvements in or relating to gas turbine engines, and in particular concerns an improved mounting of ceramic nozzle guide vanes on the engine casing.
In the operation of a gas turbine engine, air at atmospheric pressure is initially compressed and delivered to a combustion stage where heat is added to the air leaving the compressor by adding fuel to the air and burning it. The gas flow resulting from combustion of fuel in the combustion stage then expands through a turbine, delivering up some of its energy to drive the turbine and produce mechanical power, the remainder, on discharge to the atmosphere, providing a propulsive jet, in the case of the gas turbine jet engine, for example. In order to produce a driving torque, the turbine consists of one or more stages, each employing one row of stationary nozzle guide vanes and one row of moving blades mounted on or integral with a disc. The nozzle guide vanes are aerodynamically shaped to direct incoming gas from the combustion stage onto the turbine blades and thereby transfer kinetic energy to the blades.
Gases entering the turbine stage typically have an entry temperature from about 850.degree. C. to at least 1700.degree. C. Since the efficiency and power output of the turbine depend on the entry temperature of the incoming gas there is a trend in gas turbine engine technology to increase the gas temperature. A consequence of this is that the materials of which the blades and vanes are made assume ever-increasing importance with a view to resisting the effects of elevated temperature.
Nozzle guide vanes were originally made of high temperature steels or nickel alloys. More recently, it has been found that resistance to even higher gas entry temperatures may be imparted to the vanes by making them entirely of ceramic. If the nozzle guide vanes are ceramic it is important that, when they are mounted on the engine casing, which is made of metal, due regard is given to the different coefficients of expansion of the ceramic material and of the metal in the casing. It is an object of the present invention to provide a manner of mounting ceramic nozzle guide vanes on a metal engine casing so that they remain fixed relative to the engine casing despite temperature changes and do not turn about their axes when subjected to aerodynamic or other loading, otherwise the gas flow through the guide vanes onto the rotating vanes is likely to be disturbed and the efficiency of the engine adversely affected.
In this specification "radial" and "radially" will be understood to mean a direction at right angles to the longitudinal axis of the engine, and also refers to the long axis of a nozzle guide vane.